I've now made three attempts to get these clues to you privately, but my email with file attached keeps coming back undeliverable after multiple attempts by my ISP's mailer. So the best I can do is post them publicly here rather than keeping the unsourced conjectures confidential until you have a chance to investigate and possibly verify them for yourself and decide it they are valid leads or just more confusion factors that doesn't advance your search any at all.
Regarding David, son of David Abernathy and Martha/Patti Abernathy of the Bissell place. He was born no later than 1796, a grandson of James Abernathy & Elizabeth Cox. His widow'd mother is found 1800 Lincoln County Census, p. 835, Martha Abernathy, 61100-11210; Martha also found 1810 in NC. In 1820 she's listed in Lincoln County NC in Capt Rozeles militia district, p. 366, as "Abernathy, Patter, 02110-00121-06".
I do not know if this David Abernathy migrated to MO with some of his siblings and died there on 18 Mar 1847 according to a fragment of the Hager Diary, or if he was the David Abernathy said in some MO references to have died between 2nd and 11th of Feb 1846 while traveling to New Orleans. Or the David Abernathy whom you and I both know ended up in Shelby County, AL. As far as I know, he only shows up with widow'd mother as appropriately aged tally marks and I can't really be sure I estimating his age correctly. Tally marks for the children of David and Martha on the 1790 Lincoln county census allow for David as head of household, 4 males under 14 yrs of age, 4 females including Martha and one slave. I have 9 names for children of this couple, hints that there may have been 4 more, and little data on some of them, named or not. We can pinpoint the actual year of birth of only two of those named children and David is one of many where the birth year is _real_ approximate!
And here's another possibility for the David Abernathy whom you found for us in 1811 in Maury County TN. He could also be the David (Blackstone?) Abernathy who married Lavinia Townes & died and is buried in 1846 in what is now Colbert County AL, just south of the Tennessee River, one of many routes to Missouri at the time, although this David didn't migrate any further than that. I have no knowledge of the religious affiliation of any of the above David Abernathys, it being a factor I only sometimes note in my database.